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| Saturation temperature vs pressure for hydrogen, oxygen, and methane [1] |
At ambient temperature and pressure (i.e., 298 K and 1.01 bar), hydrogen is a gas with approximately 7% the density of air. This results in an advantageous rise rate of nearly 20 m/s (or six times faster than natural gas) enabling rapid dispersion of any leaks. However, this low density is a challenge for storage.
The two most commonly used methods of increasing hydrogen density in order to store significant quantities are compression and liquefaction. Compressed storage up to 700 bar is commercially available and increases the density of hydrogen by a factor of 477 times greater than ambient pressure; whereas, liquefaction increases it by 866 times compared to ambient conditions (i.e., nearly double the density of compressed hydrogen at 700 bar).
Liquefying Hydrogen
In order to liquefy hydrogen it must be cooled to a very low temperature (e.g., 20 K at 1.01 bar). This is accomplished with a cryogenic liquefier or cryocooler. Various thermodynamic cycles and equipment are available for this purpose.
All liquefaction processes are limited by the ideal Carnot efficiency which is calculated as the ratio of the cold refrigeration temperature divided by the difference in cold refrigeration and warm rejection temperatures. The actual performance of hydrogen liquefiers are a fraction of the ideal efficiency, ranging from about 30-40% of Carnot for state-of-the-art systems.
Two-stage hydrogen liquefiers generally bring the hydrogen gas down to the 80-100 K range in the first stage (i.e., sensible cooling); and then cool and liquefy it in the 20 K range in the second stage (i.e. sensible and latent cooling). Efficiencies are much higher for the first stage due to the higher refrigeration temperature.
Hydrogen liquefaction must also address the conversion of ortho-to-para hydrogen that occurs at cryogenic temperature. This change in equilibrium electron spin state is an exothermic process that is generally accelerated with a catalyst during liquefaction.
Liquid Hydrogen Storage Behavior
Insight into the behavior of hydrogen and other fluids at cryogenic temperatures can be gleaned by examining their saturation temperature at the vapor pressure of interest (see above plot). In a container of liquid hydrogen, the interface between the liquid and vapor is always at the saturation temperature corresponding to the container vapor pressure.
However, the temperatures in the hydrogen vapor space of the container - also known as the ullage - are at or above the saturation temperature (i.e., superheated). For a stationary tank, the ullage thermally stratifies with the coolest temperature near the interface and warmest temperatures near the top of the container.
Conversely, the liquid hydrogen in such a container is at or below the saturation temperature (subcooled). A stationary container with subcooled liquid will also thermally stratify with the coldest temperatures near the bottom of the tank. If the subcooled liquid is circulated toward the interface by a mixer, or from momentum forces in a mobile application, the tank pressure will drop to a new saturation condition.
Over time, the liquid hydrogen in a container will warm toward the saturation temperature (but not above it) due to heat transfer from the environment. When all of the liquid reaches saturation temperature, it will begin to boil off and raise the tank pressure. This additional vapor must either be vented when the tank pressure reaches the maximum design limit; or reliquefied to maintain "zero boil-off" storage.
Cryogenic Material Properties
Most materials behave very differently at cryogenic temperatures compared to ambient conditions. These differences must be well understood by engineers, designers, and operators of cryogenic systems.
Thermal properties of materials such as conductivity and specific heat are highly nonlinear functions of temperature in the cryogenic range. As a result, heat transfer and energy balance calculations often require integrating the property of interest over the temperature range. Simply using an average value between the upper and lower temperature can result in significant calculation errors.
Mechanical properties that can vary significantly at low temperature include:
- Yield and ultimate strength: generally increases at lower temperatures for most solids
- Ductility: some materials remain ductile (e.g. aluminum alloys, austenitic stainless steel with > 7% nickel, most face-centered cubic metals); while some materials become brittle (carbon steel, most plastics, most body-centered cubic metals)
- Elastic modulus: varies
- Fatigue strength: varies
All of the above has implications for the selection of materials in liquid hydrogen system design. Storage tanks of 300 series stainless steel are common. Aluminum alloys are also used in some applications, and titanium alloys are suitable but rarely used outside of the aerospace industry.
Seals for fittings, gaskets, and valves must be comprised of compatible elastomers for cryogenic hydrogen service. Likewise, instrumentation and sensors designed for cryogenic temperatures are required for liquid hydrogen system monitoring and process control.
In the next post I'll touch on tank design options, insulation systems, and filling/draining operations.
References
[1] Cryogenic fluid management of liquid hydrogen, oxygen, and methane: Part 1 - passive technologies, systems, and operations. Moran Innovation LLC, 2022.
